A stent is a mechanical device used to dilate and/or maintain a desired lumen in a tubular structure or duct as in the bile duct, pancreatic duct, cystic duct, ureter or urethra. Stones, contracting scar tissue or compressing neoplasia, benign or malignant, may compromise a duct lumen and are the indications for stent placement. The stent must be rigid to be forced through and dilate a narrowed area (stenosis) but sufficiently pliable to conform to the adjacent normal tissue without injuring it by erosion. Furthermore, a method must be provided to maintain the stent in place without proximal and distal migration. A stent may pass distally out of position within the duct or be lost proximally toward or into the organ. In either case, the result is failure to maintain a patent lumen and return of the obstruction and its symptoms. A passed stent must be replaced. A lost stent must be retrieved from within the organ, manipulated through the narrowing (stenosis), removed and then replaced. Beyond the technical complexity of retrieving a lost stent, the obstruction and its consequences must first be treated. These considerations emphasize the need to prevent stent loss in preference to the more easily replaced stent that has passed.
Initially, stents with pigtails on both ends (double pigtails) were used in the biliary tree including the bile and cystic ducts and the pancreas as well as the urinary tract. As technical skill and endoscopes improved, it became possible to use larger diameter stents greater than the initial 5 French (1.7 mm O.D., 1 mm I.D.). With increased stent diameter and rigidity, the pigtail exerted greater force on the wire guide used for introduction, and it became more difficult to force the stent over the wire guide through a tight stenosis without buckling. Consequently, the pigtail was replaced with a flap, particularly in stents of 10 French diameter and greater. This strategy improved introduction at the expense of the more efficient retention of a pigtail. A small flap on both ends of the stent and long stents of 12 to 15 cm used in the biliary tree prevented this displacement by both resistance of the flap to movement and length of the stent.
However, the double flap or wing stent also has disadvantages. It is difficult to select the proper length as after stent placement, and after relief of obstruction there is substantial shrinkage of the duct system that may make a straight flap stent too long and erode or gouge tissue causing pain or bleeding with premature stent obstruction. Flap stents may easily be lost into the obstructing duct during the last few forceful pushes during introduction. The relatively small wings may fracture and break off during introduction or break off due to loss of elasticity. Large 10 or 11.5 French stents are rigid and difficult to grasp for removal. This is particularly true if the stent is lost into the proximal duct system or organ cavity. The larger flaps of a 10 French or larger stent gouge or scrape the tissue on removal. This may provoke bleeding that leads to immediate occlusion of the replacement stent. These problems with flap stents are exaggerated in the pancreas where 5 and 7 French (1.7 and 2.1 mm O.D.) stents 3 to 5 cm in length are required and there is a relatively long duct (10 to 12 cm in length are required and there is a relatively long duct (10 to 12 cm) into which the stent may be lost.